1. Field of Invention
This invention relates to a method of forming siloxane polymers using a heteropoly catalyst having a Keggin structure.
2. Background Information
Many methods for forming linear siloxane polymers have been described in the art. See for example Noll, W., Chemistry and Technology of Silicones, Academic Press, Inc. 1968, pp. 209-233. In general, ring-opening polymerization of cyclodialkylsiloxanes, considered to be a chain growth process, can be initiated by either acids, such as acid clays, HF, H.sub.2 SO.sub.4, CF.sub.3 SO.sub.3 H, etc. or bases, such as GOH, GOSi.dbd., GOR, GR etc., where G is an alkali metal or quaternary ammonium or phosphonium group and R (depending on G) is an alkyl, polystyryl or poly(trimethylsilylvinyl) group. Step growth polymerization in linear polysiloxanes often consists of homocondensations of silanol-ended siloxanes catalyzed by either strong acids or bases or mild amines or carboxylic acids combined with quaternary ammonium salts. Termination of both chain growth and step growth polymerization is usually effected by neutralization of the acid or base.
Nevertheless, there are some disadvantages to these methods of forming siloxane polymers. One disadvantage is that branching, often described as "T" branching often occurs with these methods due to cleavage of R--Si groups. It has been found, for example, that siloxane polymers having an average degree of polymerization of 200 have on average about one "T" branch per polymer chain. Another disadvantage is that neutralization of the acid or base is required in order to terminate the polymerization reaction. This is an additional step which can poison the catalyst and cause additional waste products. In addition, unless all traces of acid or base are eliminated, the reverse reaction may be catalyzed. Further, the catalyst may not be easily recoverable.
The literature describes certain catalytic effects of inorganic metal-oxygen clusters, see for example Misono, M., Catal. Rev. Sci. Eng., 1987, 29, 269. These clusters are referred to in the literature as "isopolyanions" when only a polyvalent metal and oxygen are involved and can be represented by the formula (M.sub.m O.sub.y).sup.p-. If an additional metallic or non-metallic element is present, these clusters are referred to as "heteropolyanions" and can be represented by the formula (X.sub.x M.sub.m O.sub.y).sup.q-.
In the formulae described above, M is a polyvalent metal such as molybdenum, tungsten and less frequently vanadium; X is the "heteroatom" and can be almost any element in the periodic table, other than a noble gas; O is oxygen; x, m, and y are integers where x&lt;&lt;m and p and q represent the charge on the anion. When x=1, m=12 and y=40, the heteropolyanion usually adopts what is known as the Keggin structure. With this Keggin structure, the central heteroatom in a XO.sub.4 tetrahedron is surrounded by 12 MO.sub.6 octahedra that share edges and corners to form a heteropolyanion with an overall tetrahedron symmetry.
The heteropolyanions are typically negatively charged and so easily associate with various cations, including hydrogen, alkali metals, alkaline earth metals and ammonium cations. When hydrogen is the cation, heteropoly acids are formed. Salts may be formed by adding any of the other types of cations described above and acid-salt mixtures are obtained when both hydrogen and other types of cations are present. The addition of appropriate amounts of the various cation(s) having positive charges will balance the negative charge of the anion so that the overall valence of the heteropoly compound is zero. These neutral heteropoly compounds when m is 12 and y is 40 are referred to herein as "heteropoly catalysts having a Keggin structure" or "heteropoly catalysts."
These heteropoly catalysts having a Keggin structure are particularly suitable for catalysis reactions because they are stable in solution and in solid form. Large scale industrial processes using heteropoly catalysts, include, hydration of propene, oxidation of methacrolein and polymerization of tetrahydrofuran.
The main advantages of heteropoly catalysts and heteropoly acids in particular, over traditional protic acids such as sulfuric acid, hydrochloric acid, hydrofluoric acid, etc are their versatility towards catalytic molecular design and their low environmental hazard.
The former advantage is due to the heteropoly catalyst having at least three parts a cationic part, an anionic part and a solvation sphere. Altering any of these parts can produce a catalyst having different surface activity, charge and reactivity.
In addition to their versatility, heteropoly catalysts are important because they are easily recovered, recrystallized if necessary, and reused. The catalysis can be terminated at will without having to use neutralization steps that can poison the catalyst and create additional waste products.
One of the objectives of the method of the present invention is the preparation of siloxane polymers from siloxane polymer precursors which have less branching than siloxane polymers produced by other methods. Another objective of the method of this invention is to form siloxane polymers using a catalyst which does not require neutralization.